Organic pigments are generally more durable than dyes and have higher coloring power than inorganic pigments. Thus, they are widely used as coloring materials for printing inks, paints, toners, color filters and the like, and various manufacturing methods are known.
For example, a manufacturing method in which pulverization treatment is performed in an organic solvent has been proposed, as disclosed in Patent Literature 1. However, by such a method, there are problems that crystallinity of the intended organic pigment microparticles is lowered, a large amount of power is required, inclusion of impurities is inescapable, and high cost is required.
As one of methods for producing organic pigment microparticles which have solved these problems, there is a method of precipitating organic pigment microparticles by mixing an organic pigment solution in which the organic pigment is dissolved in a good solvent capable of dissolving the organic pigment and a poor solvent having a low solubility of the organic pigment, as disclosed in Patent Literature 2. However, there are problems that the organic pigment microparticles prepared by the method as described in Patent Literature 2 often contains an amorphous component, and the pigment particles grow and/or aggregate in a dispersion medium such as water or an organic solvent.
In general, the coloring power is improved by micronizing organic pigments, but at the same time the crystallinity tends to be lowered. In the case where distribution of the particle diameter is wide, the finer the particles are, the more necking tends to occur due to Ostwald ripening. It is difficult to exhibit the expected original characteristics of organic pigment microparticles, such as inherent colors, durability, transmission characteristics and the like.
Therefore, as known from Patent Literature 3, it is conceivable to use a derivative of an organic pigment as a crystal growth inhibitor. However, when a derivative of an organic pigment is used as a crystal growth inhibitor, there is a serious disadvantage that the characteristics of the organic pigment, particularly the inherent colors, change.
Therefore, as known from Patent Literature 4, it is more desirable as a technique for suppressing crystal growth without changing the characteristics of the organic pigment to coat organic pigment microparticles with, for example, an oxide such as silica and the like after precipitation of the organic pigment microparticles.
However, in Patent Literature 4, since it is manufactured in a batch system, step of precipitating organic pigment microparticles and step of coating the precipitated organic pigment microparticles with an oxide are separated. For this reason, there is a disadvantage that a part of the precipitated organic pigment microparticles grows or aggregates before transition to the oxide coating step, and as a result, the coarsened organic pigment microparticles are coated with the oxide. Therefore, it is still difficult to exhibit the expected original characteristics of the organic pigment microparticles.
On the other hand, Patent Literature 5 proposed by the present applicant, discloses that precipitation of organic pigment microparticles and introduction of a modifying group into the precipitated organic pigment microparticles are continuously performed in the thin film fluid between the processing surfaces which are disposed so as to face each other, being capable of approaching to and separating from each other, at least one of which rotates relatively to the other.
However, in Patent Literature 5, there remains a problem that the characteristics of the pigment change due to the chemical reaction by introducing the modifying group. Furthermore, the precipitated organic pigment microparticles are not coated with an oxide, and the organic pigment microparticles produced according to Patent Literature 5 still have a problem of coarsening in the dispersion medium. Therefore, it is required that growth and/or aggregation of particles are suppressed, dispersibility and solvent resistance are improved, and thereby the characteristics such as the inherent colors of the organic pigment microparticles and durability are sufficiently exerted by coating at least a part of the organic pigment microparticles after precipitation of the organic pigment microparticles with an oxide coating prior to growth and/or aggregation.